Recently, in dental hospitals and the like, a method of directly acquiring three-dimensional shape data (i.e., an image) inside the oral cavity of a patient by using a scanner, and then processing a prosthesis, which is applied to the oral cavity of the human body, by using the acquired three-dimensional shape data has been used. This method is referred to a method of processing a prosthesis by using computer aided design (CAD)/computer aided manufacturing (CAM).
However, a very large size of scanner is required to acquire three-dimensional shape data in the entirety of the oral cavity of a patient at once, and in this case, it is difficult to actually insert such a big scanner into the oral cavity, and even if such a big scanner can be inserted into the oral cavity, it causes a patient to feel much inconvenience. Thus, three-dimensional shape data is generally acquired by inserting a scanner with a small size into the oral cavity and three-dimensionally scanning an internal structure of the oral cavity while continually changing a position of the scanner to acquire a plurality of scanning data according to position, and then composing (i.e., combining) these data. However, when the internal structure of the oral cavity is scanned with a scanner, a distance and angle between the scanner and a subject are continually changed, and thus in CAD, respective images are compared with each other and combined according to a combining scheme based on a relative coordinate system to form a three-dimensional structure, but the formed three-dimensional structure is considerably different from the actual subject. That is, for composing or combining scanning data for respective positions, it is necessary to adjust, to a predetermined size, a size of each of images realized by the scanning data, and then directly connect a scanning datum for any one of the positions to a scanning datum for a position adjacent thereto. However, conventionally, such a composing or combining method is virtually impossible, and thus finally acquired three-dimensional shape data is not substantially inconsistent with an actual internal structure of the oral cavity. Therefore, a prosthesis manufactured by using the three-dimensional shape data also has a different shape from that of an actual internal structure of the oral cavity, resulting in dental aftereffects such as malocclusion, tooth deformation, or a headache.